1. Field of the Invention
The present invention relates to a stabilizer control apparatus for a vehicle, and more particularly to an apparatus for controlling a torsional rigidity of a stabilizer disposed between a right wheel and a left wheel, by means of an electrically operated actuator.
2. Description of the Related Arts
In general, a stabilizer control apparatus for a vehicle is provided for applying an appropriate roll moment to a vehicle by means of a stabilizer while the vehicle is performing a turning operation, to reduce or restrict a rolling motion of the vehicle body. In order to achieve this function, a system called “Dynamic Drive” has been proposed to reduce the rolling motion by actively using hydraulic pressure, in a Technology Report of BMW as retrieved on Dec. 8, 2003 from Internet (URL: http://www.bmwgroup.com/e/0—0_www_bmwgroup_com/7_innovation/7—3 _technologie/7—3—4_dynamic_drive.shtml). In that report, it is stated that two pressure control valves are responsible for regulating the required pressure and a pilot valve determines the direction of the counter-torque moments (left-hand or right hand curve). To ensure that the valves operate optimally, the controller evaluates the pressure signal for the front and rear axle stabilizers, the lateral acceleration signal, and the CAN signals reporting both longitudinal and lateral movements of other controllers (DSC etc.). By evaluating the CAN signals, the Dynamic Drive controller obtains a great amount of additional information which it uses to test the plausibility and selection of the incoming signals.
Also, an active roll restraining control apparatus using an electric system has been known heretofore. For example, Japanese Patent Laid-open Publication No.2000-71739 discloses an apparatus for controlling efficiency of a stabilizer to vary an apparent torsional rigidity of the stabilizer by driving and controlling an actuator in response to a turning level of a vehicle. In practice, driving force of an electromagnetic linear actuator is calculated on the basis of signals of various sensors, and converted into electric value to provide a desired electric value for performing a PID control. And, it is described that the actuator is actuated to enlarge or shorten the stabilizer, so as to provide an appropriate torsional rigidity for it, by feeding exciting current to a stator having laminated plates with coils connected together in a three-phase delta circuit, in response to a synchronous signal based on the output of position detecting means, and feeding actual current back to it.
Furthermore, in the U.S. Pat. No. 6,425,585 (corresponding to International Publication No. WO9967100, and Japanese Patent Laid-open Publication No. 2002-518245), there is disclosed a system for stabilizing vehicles against rolling, with at least one slewing drive arranged between halves of the front and/or rear chassis stabilizer, thus creating an initial stress of the stabilizer halves to reduce or suppress the rolling motion and, in the event of roll, applying a counter-torque to the vehicle body as a function of output signals of a sensor for detecting a roll parameter. The slewing drive includes three basic components, namely an electric motor, a step-down gear and a brake disposed between them. The torque generated by the electric motor is converted by the step-down gear into the torque needed for the initial stress of the stabilizers. One stabilizer half is via a bearing mount connected directly to the casing of the electromechanical slewing drive and the other stabilizer half is connected to the output end (high torque end) of step-down gear and is mounted in the bearing mount.
It can be said that the Dynamic Drive system as described above relates to an active roll restraining control apparatus using an electric-hydraulic pressure system, which controls a proportional pressure valve and directional control valve in response to a signal of a pressure sensor, to perform a so-called pressure control. According to the apparatus for restraining the roll actively by means of the stabilizer, in order to restrain the rolling motion when the vehicle is turning, a roll moment is provided by controlling the stabilizer, against the roll moment created by inertia force applied to the vehicle body during the turning operation. Therefore, it is required to provide the pressure for the stabilizer actuator, which is converted directly into the roll moment, to perform a feed-back control.
On the other hand, in the Japanese Patent Laid-open Publication No.2000-71739, there is disclosed an embodiment of the apparatus which is provided with the electromagnetic linear actuator to be moved linearly, between the stabilizer bars and suspension members. However, any other practical examples of the actuator have not been disclosed. Particularly, it is silent about the actuator having the electric motor and the step-down gear as disclosed in the U.S. Pat. No. 6,425,585, However, in the case where the force output from the electric motor is transmitted via the step-down gear, which act as a speed reducing mechanism, a problem may be caused as follows. That is, the speed reducing mechanism provided for the actuator has a normal efficiency for transmitting the force from the electric motor to the vehicle body, and a reverse efficiency for transmitting the force output from vehicle body to the electric motor. Due to the efficiency (normal efficiency and reverse efficiency) of the speed reducing mechanism, if the active roll moment is provided for the roll restraining control on the basis of a lateral acceleration (actual lateral acceleration, or calculated lateral acceleration), the rolling motion may not be converged properly.